In view of the recent increasing concern about energy saving and environmental problems, great attention is focused on hybrid vehicles. A hybrid vehicle employs a battery, an inverter, and a motor driven by the inverter, as well as a conventional engine, as the motive power source. In addition to obtaining motive energy by driving the engine, the voltage from the battery is converted into alternating voltage by the inverter, and the motor is rotated by the converted alternating voltage to obtain further motive energy.
In such a hybrid vehicle, the charge and discharge of a battery is controlled such that the amount of charge indicating the state of charge (SOC) of the battery (hereinafter, also simply referred to as “SOC”; represented by 0 to 100%) falls within a predetermined range.
It is known that such battery charging and discharging within a predetermined range will cause an error between the SOC estimated value and the actual SOC, reduction in the charging/discharging capacitance by the so-called memory effect, degradation by sulfation, and the like. These problems can be solved by carrying out the so-called refresh on the battery. In other words, these problems can be eliminated by achieving substantially a fully-charged or fully-discharged state of the battery. It is known that the refresh effect is increased by charging or discharging the battery such that the SOC greatly deviates from the control range.
Japanese Patent Laying-Open No. 2004-328906 discloses a charging control apparatus for a hybrid vehicle that carries out such battery refresh. In this charging control apparatus, the continuous operating time of the engine is calculated based on the vehicle state. Furthermore, the time required for refresh-charging the battery is calculated. Refresh-charging of the battery is effected when determination is made that the engine continuous operation time is longer than said time required for charging.
Since refresh charging is completed before or simultaneous to the elapse of the engine continuous operation time by the charging control apparatus, the engine will not be driven just for the purpose of refresh-charge. As a result, reduction in mileage can be suppressed.
However, the charging control apparatus disclosed in Japanese Patent Laying-Open No. 2004-328906 is disadvantageous in that fuel consumption by the engine is encountered since the generator is driven by the engine to carry out refresh-charging of the battery.
Particularly in a hybrid vehicle that can have the battery charged by means of a power source external to the vehicle, the SOC control range will become higher than that of a hybrid vehicle absent of an external charging function. There is a possibility that sufficient refresh effect cannot be achieved even if refresh-charging is carried out.